Check source.
This calibrated beta source is intended for checking the operation of Geiger counters and the like. Sale of buttons like this would ordinarily be fairly regulated, but this one neatly avoids the issue by being constructed out of several antique radium paint watch hands, which are widely available and generally speaking legal:

The seller sternly warns against removing the cover paper, which reveals the watch hands. I'm not sure if this is mainly because they don't want people to know that this is how the button is made, or mainly because they are genuinely worried about the hazard of exposure to radium paint chips and dust. In any case, I would head their warnings: Radium, like any alpha emitter, is much safer in a well-sealed container than it is loose, because if any gets in your lungs it will tend to stay there until you die of lung cancer.Source:eBay seller geoelectronicsContributor:Theodore GrayAcquired:20 November, 2003Price: $20Size: 1"Purity: <1%